At the onset it should be noticed that the contents of this chapter are confined to the fusion of protoplasts; normal somatic plant cells with thick cell walls cannot fuse with each other and only naked protoplasts can do so. Nowadays fusion of protoplasts has attracted attention as the first step towards somatic hybridization, but the phenomenon is also interesting from the view of plant physiology. The first report of the fusion of protoplasts was by KuSter (1909, 1910). He observed the fusion of protoplasts which were isolated from the epidermis of tulip bulbs according to the mechanical method of KLercker (1892). Later MIchel (1937) observed the fusion between protoplasts from different species and tissues. These protoplasts were also isolated mechanically from the tissues plasmolyzed previously with 0.5 M KNO3; however, when sucrose was employed as a plasmolyticum, fusion was not observed. In these pioneering studies the work of MIchel is especially important, because fusion could be induced not only between the protoplasts from the same tissue, but also between protoplasts from different tissues and different plants. However, these interesting observations went unnoticed until the report of POwer et al. (1970). This neglect may be due to the fact that at the time of MIchel the technique of plant cell culture had not yet been established and the yield of protoplasts from tissues was extremely restricted because of the mechanical method utilized. The new possibility for the development of protoplast research started from the report of COcking (1960), who isolated protoplasts from root tissues of tomato by a self–made crude cellulase preparation from a fungus, Myrothecium verrucaria.